This invention relates to the fabrication of composite materials, and, more particularly, to the fabrication of a particle-reinforced composite material by thermal spray processing.
A number of metal silicides, boride, and carbides have great potential as coatings or freestanding structural materials for use in elevated-temperature applications. However, these materials typically exhibit a low fracture toughness at room temperature, low thermal shock resistance, and low creep resistance at elevated temperatures of greater than about 1100xc2x0 C. These mechanical properties inhibit the utilization of the materials in otherwise attractive applications.
As discussed in the parent application Ser. No. 09/260,395, now U.S. Pat. No. 6,106,903, the mechanical properties of one of the members of this group, molybdenum disilicide, may be significantly improved by forming a composite material of particles of silicon carbide dispersed throughout the molybdenum disilicide. Such composite materials prepared by powder compaction and sintering techniques have exhibited improved room temperature toughness and elevated temperature strength. The presence of the silicon carbide also reduces the coefficient of thermal expansion of the composite material as compared with monolithic molybdenum disilicide. Powder techniques, however, are not practical for many applications, such as certain types of coatings and large freestanding structures.
Other fabrication techniques for composites of molybdenum disilicide and silicon carbide have been proposed. For example, U.S. Pat. No. 5,472,487 discloses the loose mixing of molybdenum disilicide and any of several other types of powders, silicon carbide being one of the disclosed other powders. This loose mixture of separated particles is applied by low pressure plasma spraying of the loose mixture. The present inventor has recognized that this disclosed approach may be well suited for the fabrication of some types of composite materials, but is of limited value in preparing a composite material containing silicon carbide, because of the elevated-temperature sublimation of silicon carbide from the solid state to the gaseous state during the low pressure plasma spraying. The sublimation of the silicon carbide results in its loss from the mixture, so that the amount of silicon carbide in the final product is substantially lower than in the starting material.
The parent application provided a solution to the problems associated with using the metal borides, silicides, and carbides in the specific case of molybdenum disilicide. There is a need to extend this solution to the more general case. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method for preparing a particle-reinforced composite material. A wide range of volume fractions of reinforcement particles in the composite material may be prepared. Little if any of the reinforcement material is lost in the deposition procedure, so that the final product has about the same volume fraction of reinforcement material as the starting material. The composite material is substantially fully dense, with few if any voids therein. The approach of the invention may be utilized to fabricate both coatings and freestanding structures. Large articles may be prepared relatively inexpensively, without the need for large containment chambers and the like.
In accordance with the invention, a method for preparing a mass of a thermally sprayed composite material comprises the steps of providing a precomposited powder comprising a plurality of powder particles, and thereafter thermal spray depositing the precomposited powder at an ambient pressure of no less than about 0.75 atmosphere in an oxidation-preventing atmosphere, to form a thermal sprayed mass. Each powder particle comprises a matrix having a matrix composition comprising at least one matrix chemical combination of a matrix metal and a matrix non-metal selected from the group consisting of silicon, boron, and carbon, and mixtures thereof. Each powder particle further comprises a plurality of reinforcing particles, the reinforcing particles being distributed within and encapsulated by the relatively larger matrix and having a reinforcement-particle composition selected from the group consisting of silicon carbide, boron carbide, silicon nitride, and boron nitride.
The matrix metal is preferably selected from the group consisting of hafnium, zirconium titanium, vanadium, niobium, tantalum, and tungsten, and mixtures thereof. The preferred matrices prepared by this approach include silicides such as hafnium silicide, zirconium silicide, titanium silicide, vanadium silicide, niobium silicide, tantalum silicide, and tungsten silicide; borides such as hafnium boride, zirconium boride, titanium boride, vanadium boride, niobium boride, tantalum boride, and tungsten boride; and carbides such as hafnium carbide, zirconium carbide, titanium carbide, vanadium carbide, niobium carbide, tantalum carbide, and tungsten carbide.
The thermal sprayed mass typically comprises from about 5 volume percent to about 60 volume percent, more preferably from about 10 volume percent to about 50 volume percent, of the reinforcing particles, balance the matrix (plus any other constituents present).
The precomposited powder preferably comprises relatively finer reinforcement particles, preferably having a particle size of from about 0.1 micrometer to about 1 micrometer, distributed within and encapsulated by relatively coarser powder particles of the matrix composition, preferably having a particle size of from about 5 to about 80 micrometers. Such precomposited powder may be prepared using high temperature self-sustaining combustion synthesis or any other operable technique.
The thermal spraying is preferably accomplished by plasma spraying, most preferably argon-shrouded plasma spray deposition at 1 atmosphere ambient pressure. The thermal spraying may instead be accomplished in an environmental chamber with a protective atmosphere of argon or other oxidation-preventing gas. The argon-shrouded plasma spray approach is preferred because large areas or parts may be prepared without the expense of a correspondingly sized environmental chamber. The thermal spraying is typically accomplished by depositing the thermally sprayed precomposited powder onto a substrate, such as a surface to be coated or a form for a freestanding article. The thermal spray approach is relatively economical for fabricating large areas or structures.
After thermal spraying, the thermal sprayed mass may optionally be heat treated to stress relieve internal stresses within the mass. Such internal stresses, where present and not relieved, may promote the premature failure of the thermal sprayed mass during thermal excursions or in other circumstances. The heat treatment is typically accomplished at a temperature of from about 800xc2x0 C. to about 1400xc2x0 C.
The present processing approach is carefully selected in order to fabricate the desired composite thermal sprayed mass. The precomposited powder must be used. The powder cannot be thermally sprayed as a loose mixture with separated particles of the matrix material and the reinforcement particles, as suggested by the ""487 patent, because the reinforcement particles may sublime or otherwise degrade at elevated temperature. In that case where separated powders are used, a portion of the reinforcement may be lost as a vapor, and cannot be properly plasma sprayed because it is never present as a liquid phase that may bond with the matrix material. In the precomposited powder used in the present invention, the smaller, volatile reinforcement particles, and/or nitrides susceptible to decomposition such as silicon nitride, are encapsulated within the larger powder particles of the matrix phase, so that the liquification required for the successful thermal spraying is accomplished by the matrix material.
Further, the precomposited powder is applied by thermal spraying at about 0.75 atmosphere or greater ambient pressure, preferably at from about 0.75 atmosphere to about 1.25 atmospheres ambient pressure, and most preferably at 1 atmosphere ambient pressure. Spray fabrication of separated silicon carbide powder at greatly reduced pressures, as in the low-pressure plasma spray process used in the ""487 patent, results in sublimation and at least partial evaporative loss of the silicon carbide. The combination of the use of precomposited powder and a spray process operating at about 0.75 atmospheres or greater pressure results in very little loss of the silicon carbide during application. Typically, the thermal sprayed mass has a volume percent of reinforcement particles that is no greater than 5 percentage points less than a volume percent of the reinforcement particles in the precomposited powder. For example, if the precomposited powder has about 45 volume percent of silicon carbide, the thermal sprayed mass would also have about 45 volume percent of silicon carbide, and in any event typically not less than about 40 volume percent of silicon carbide.
The avoidance of substantial loss of a volatile constituent during the thermal spraying operation is an important advantage of the present invention. Thermal sprayed composite masses with relatively large volume fractions of silicon carbide may be readily prepared. In reduced-pressure thermal spray processes using separated powders, by contrast, the maximum amount of a volatile constituent such as silicon carbide that may be incorporated in the final product is usually limited to less than about 10 volume percent due to the evaporation. Additionally, with the present approach it is not necessary to clean up substantial amounts of sublimed and evaporated silicon carbide from chamber walls, pumps, and the like as in the case of reduced-pressure spray processes.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.